1. Field of the Invention
The present invention relates to an optical integrated circuit element usable for optical communications, optical information processing, optical sensing, and the like. More specifically, the present invention relates to a waveguide type optical integrated circuit element where a semiconductor laser which acts as a light emitting device and an optical waveguide for propagating light output from the semiconductor laser are integrally formed on a same semiconductor substrate, and a method for fabricating such a waveguide type optical integrated circuit element.
2. Description of the Related Art
With the present rapid progress in multimedia society, it is anticipated that optical communications with a large capacity and a high speed of 100 Mbps or more will become available at home in near future. In particular, the development of wireless optical transmission technology not only makes wirings for communications unnecessary, but also provides a great benefit in realizing a communication link using a portable computer via a terminal at a nearby relay point.
FIG. 6 shows an example of a conventional waveguide type optical integrated circuit element used as a receiver section of a wireless optical communication system.
The wireless optical communication system adopts a heterodyne wave detection method where frequency-modulated signal light 612 is combined with locally oscillated light 611 in the receiver section, to be converted into a beat signal having a frequency identical to the difference frequency. This method is advantageous over a general intensity modulation direct detection method in that the communication is excellent because a good signal to noise characteristic can be realized.
Referring to FIG. 6, the configuration of the conventional waveguide type optical integrated circuit element will be described together with the operation thereof. The waveguide type optical integrated circuit element includes a semiconductor laser 200 and two combinations of optical waveguides 630, 631 and 632, 633, which are integrally formed on a same substrate 100. An optical branching element 620 is also integrally formed at the crossing of the two combinations of optical waveguides 630, 631 and 632, 633.
The locally oscillated light 611 emitted from the semiconductor laser 200 is introduced into the input-side optical waveguide 630 among the integrally-formed optical waveguides. The light is then branched into two by the optical branching element 620 to be introduced into the output-side optical waveguides 631 and 633.
On the other hand, the transmitted signal light 612 is introduced into the input-side optical waveguide 632. The light is then branched into two by the optical branching element 620 to be introduced into the output-side optical waveguides 631 and 633. As a result, the locally oscillated light 611 and the signal light 612 are combined in the output-side optical waveguides 631 and 633, so as to obtain beat signals.
In the fabrication of the waveguide type optical integrated circuit element with the above configuration, it is required to form the semiconductor laser and the optical waveguide integrally on a same substrate. One example of the method for realizing this integration is an abutting method as shown in FIG. 7A. Referring to FIG. 7A, which shows an ideal integration by the abutting method, a distributed feedback (DFB) type semiconductor laser 200 formed on a semiconductor substrate 100 is vertically etched to remove part thereof, and an optical waveguide structure 300 is formed in the etched area. The optical waveguide structure 300 includes an optical waveguide layer 306, optical confinement layers 304 and 308 sandwiching the optical waveguide layer 306, a buffer layer 302, and a capping layer 309 located on the outer sides of the optical confinement layers 304 and 308, respectively. The semiconductor laser 200 includes a first cladding layer 202, an active layer 204, a carrier barrier layer 205, a first guiding layer 206, a second guiding layer 207, and a second cladding layer 208. Light emitted from the semiconductor laser 200 is directly coupled with the optical waveguide structure 300, and propagates in the optical waveguide layer 306.
The abutting method described above eliminates the necessity of positioning the semiconductor laser and the optical waveguide with each other, thereby providing high mechanical stability, compared with a method where they are separately fabricated and then bonded together.
The above conventional method is disadvantageous in the following points.
(1) In reality, the optical waveguide structure is not formed as ideally shown in FIG. 7A in the area formed by the vertical etching, but is formed as shown in FIG. 7B, for example. That is, the optical waveguide layer 306 of the optical waveguide structure 300 is slanted from the horizontal direction in the interface area with the semiconductor laser 200. In such a slant layer area, since light is influenced by the refractive index distribution in the area, the percentage of light which is not coupled with the optical waveguide layer 306 increases. Thus, the coupling ratio is much lower than that anticipated from the ideal configuration.
(2) When the vertical beam diameter of light emitted from the semiconductor laser 200 does not match with the vertical beam diameter in a native mode of the optical waveguide structure 300, the greater the difference therebetween, the lower the percentage of light emitted from the semiconductor laser 200 which is coupled with the optical waveguide structure 300 becomes.
The above problems (1) and (2) will be described more specifically.
FIG. 7B shows the case where a GaAs/AlGaAs DFB semiconductor laser is vertically etched and then AlGaAs materials are grown in the etched area by metal organic chemical vapor deposition (MOCVD) to form the optical waveguide structure 300.
In the process of the growth of the AlGaAs materials, since the growth rate greatly depends on the plane orientation, a plane with a lower growth rate grows more slowly than a plane with a higher growth rate, resulting in the structure as shown in FIG. 7B. In this case, a slant layer structure slanted from the horizontal direction is formed at the interface between the semiconductor laser 200 and the optical waveguide structure 300. Accordingly, part of light emitted from the semiconductor laser 200 is reflected or refracted by the slant layer structure at the interface, thereby to be radiated outside the optical waveguide structure, not coupled with the optical waveguide layer 306. In other words, radiation loss occurs.
It has been confirmed from the results of experiments conducted by the inventors of the present invention that light of about 1 dB was radiated by the slant layer structure at the interface. The inventors fabricated various types of the optical waveguide structure under various different conditions. The resultant configurations of the optical waveguide structures varied depending on the conditions, but it was not possible to obtain the ideal configuration as shown in FIG. 7A. In any case, a radiation loss in the range of 0.5 to 1 dB was observed.
Moreover, in the conventional case, the thickness of the optical waveguide layer 306 of the optical waveguide structure 300 was about 2 xcexcm while the vertical beam diameter of the semiconductor laser 200 was about 1 xcexcm. This difference caused a great mode mismatch when light emitted from the semiconductor laser 200 was coupled with the optical waveguide layer 300. Due to this mode mismatch, a radiation loss of 1.7 dB was observed.
Thus, the total radiation loss amounts to about 2.7 dB. Due to this radiation loss, the semiconductor laser 200 is forced to provide a light output higher than that actually required. As a result, the power consumption of the semiconductor laser 200 increases, and moreover the reliability of the semiconductor laser 200 is reduced.
The waveguide type optical integrated circuit element of this invention includes: a semiconductor laser of an end face output type; and an optical waveguide for propagating output light from the semiconductor laser, the optical waveguide including a plurality of semiconductor layers, the semiconductor laser and the optical waveguide being integrally formed side by side on a semiconductor substrate, wherein a single semiconductor layer is buried in an interface area between the semiconductor laser and the optical waveguide.
In one embodiment of the invention, the single semiconductor layer has a width of about 20 xcexcm.
Alternatively, the waveguide type optical integrated circuit element of this invention includes: a semiconductor laser of an end face output type; and an optical waveguide for propagating output light from the semiconductor laser, the optical waveguide including a plurality of semiconductor layers, the semiconductor laser and the optical waveguide being integrally formed side by side on a semiconductor substrate, wherein a semiconductor layer of which the refractive index is substantially continuously changed in a thickness direction is buried in an interface area between the semiconductor laser and the optical waveguide.
In one embodiment of the invention, the refractive index of the semiconductor layer changes parabolically where the refractive index is higher at a position closer to the center of the semiconductor layer in the thickness direction.
In another embodiment of the invention, the refractive index is changed by changing the composition of the semiconductor layer.
In still another embodiment of the invention, the center of the semiconductor layer in the thickness direction matches with the center of an output light distribution of the semiconductor laser and the center in a native mode of the optical waveguide.
In still another embodiment of the invention, a single semiconductor layer is buried at at least one of an interface between the semiconductor layer and the semiconductor laser and an interface between the semiconductor layer and the optical waveguide.
Alternatively, the waveguide type optical integrated circuit element of this invention includes: a semiconductor laser of an end face output type; and an optical waveguide for propagating output light from the semiconductor laser, the optical waveguide including a plurality of semiconductor layers, the semiconductor laser and the optical waveguide being integrally formed side by side on a semiconductor substrate, wherein a dielectric layer is formed in an interface area between the semiconductor laser and the optical waveguide.
In one embodiment of the invention, dielectric layers are interposed between the semiconductor layer and the semiconductor laser and between the semiconductor layer and the optical waveguide.
In another embodiment of the invention, the semiconductor laser is a distributed feedback type semiconductor laser.
According to another aspect of the invention, a method for fabricating a waveguide type optical integrated circuit element is provided. The method includes the steps of: forming a semiconductor layer constituting a semiconductor laser on a semiconductor substrate; removing a portion of the semiconductor layer corresponding to a first region by etching so as to have a substantially vertical section; forming a semiconductor layer constituting an optical waveguide in the first region; removing a portion including an interface between a light output end face of the semiconductor laser and a light incident face of the optical waveguide corresponding to a second region by etching so as to have a substantially vertical section; and forming a single semiconductor layer in the second region.
Alternatively, the method for fabricating a waveguide type optical integrated circuit element of this invention includes the steps of: forming a semiconductor layer constituting a semiconductor laser on a semiconductor substrate; removing a portion of the semiconductor layer corresponding to a first region by etching so as to have a substantially vertical section; forming a semiconductor layer constituting an optical waveguide in the first region; removing a portion including an interface between a light output end face of the semiconductor laser and a light incident face of the optical waveguide corresponding to a second region by etching so as to have a substantially vertical section; and forming a semiconductor layer of which the refractive index is substantially continuously changed in a thickness direction in the second region.
Alternatively, the method for fabricating a waveguide type optical integrated circuit element of this invention includes the steps of: forming a semiconductor layer constituting a semiconductor laser on a semiconductor substrate; removing a portion of the semiconductor layer corresponding to a first region by etching so as to have a substantially vertical section; forming a dielectric mask on the section of the semiconductor layer; and forming a plurality of semiconductor layers constituting an optical waveguide in the first region.
In one embodiment of the invention, the step of forming a dielectric mask includes employing a bias sputtering method where sputtering is performed at the same time when a bias voltage is being applied to the semiconductor substrate.
Alternatively, the method for fabricating the waveguide type optical integrated circuit element includes the step of: matching the center of the semiconductor layer in the thickness direction with the center of the output light distribution of the semiconductor laser and the center in the native mode of the optical waveguide by controlling the flow obtained by a mass flow controller of an MOCVD apparatus.
Thus, according to one embodiment of the waveguide type optical integrated circuit element of the present invention, a single semiconductor layer is buried in the interface area between the semiconductor laser and the optical waveguide. Accordingly, a slant layer structure does not exist in the interface area.
This makes it possible to reduce the difference in the equivalent refractive index at the interface between the semiconductor laser and the buried region and at the interface between the buried region and the optical waveguide. At such interfaces, the propagating light is hardly reflected nor refracted, allowing for a reduction in the radiation loss.
Moreover, with the above configuration, the beam diameter of the output light from the semiconductor laser can be easily matched with the beam diameter in the native mode of the optical waveguide. This suppresses the radiation loss due to the mode mismatch.
Thus, according to the present invention, since the coupling loss can be greatly reduced, a waveguide type optical integrated circuit element with reduced power consumption and improved reliability can be realized.
In another embodiment of the invention, a semiconductor layer of which refractive index is substantially continuously changed in the thickness direction is buried in the interface area between the semiconductor laser and the optical waveguide. Accordingly, the mode profile of light propagating in the semiconductor layer continuously changes due to the lens effect of the semiconductor layer, so that the light is coupled with the optical waveguide when the mode profile matches with the beam diameter in the native mode of the optical waveguide. Thus, the coupling loss due to the mode mismatch can be more effectively reduced.
When the above two configurations are combined, the effects of the two configurations can be synergically obtained to provide a waveguide type optical integrated circuit element which can effectively reduce the coupling loss further.
In still another embodiment of the invention, a dielectric layer is formed in the interface area between the semiconductor laser and the optical waveguide. Also with this configuration, a waveguide type optical integrated circuit element which does not include a slant layer structure in the interface area is realized. Thus, the coupling loss can be reduced.
In still another embodiment of the invention, a dielectric layer is formed in the interface area between the semiconductor laser and the optical waveguide, and a semiconductor layer where the refractive index is substantially continuously changed in the thickness direction is buried in the interface area between the semiconductor laser and the optical waveguide. With this configuration, the slanted growth of the semiconductor layers is prevented, and the mode mismatch is eliminated. By obtaining these effects synergically, a waveguide type optical integrated circuit element which can greatly reduce the coupling loss is realized.
Thus, the invention described herein makes possible the advantages of (1) providing a waveguide type optical integrated circuit element which can minimize light loss at the interface between a semiconductor laser and an optical waveguide, thereby reducing the power consumption and improving the reliability, and (2) providing a method for fabricating such a waveguide type optical integrated circuit element.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.